22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Pulsed surface wave discharge in helium: time evolution of the plasma column
and its gas temperature
A. Hamdan1, F. Valade1, J. Margot1, F. Vidal2 and J.-P. Matte2
Université de Montréal, Groupe de physique des plasmas, C.P. 6128, succ. centre-ville, Montréal, Québec, H3C 3J7,
Canada
2
Institut National de la Recherche Scientifique (INRS) Centre Énergie, Matériaux et Télécommunications, 1620 boul.
Lionel Boulet, Varennes, Québec, J3X 1S2, Canada
1
Abstract: A pulsed surface wave discharge in helium at intermediate pressure is presented.
The time-evolution of the plasma column shape is studied by using iCCD imaging. It has
been observed that a ‘plasma bullet’ can form itself several tens of microseconds after
breakdown. On the other hand, by using the emission of OH molecular band, the gas
(rotational) temperature was derived as a function of time and at different axial positions.
Keywords: Pulsed plasma, surface wave discharge, iCCD imaging, and gas temperature.
1. Introduction
Plasmas created by electromagnetic surface waves
(SWDs) were discovered in the seventies [1]. These
plasma sources have been the subject of many theoretical
and experimental studies, and have led to the
development of applications in material processing [2],
gas treatment and reforming [3], lighting [4], sterilization
[5], etc. Despite these well-developed applications, the
physics of these discharges during the breakdown phase is
not fully understood yet. Preliminary studies of pulsed
SWDs (PSWDs) were performed in the 80’s [6].
Recently, the plasma column dynamics was shown to
depend of the operating frequency and of the pulse-width
of the microwave power [7]. Indeed, the plasma shape is
bullet-like at low operating frequency (< 1 kHz typically)
and becomes homogeneous long-column at high operating
frequency (> 10 kHz typically).
In order to explore the physics that governs discharge
ignition and plasma plume propagation, we study in this
paper PSWDs in helium gas at intermediate gas pressures.
The discharge is characterized by time resolved
techniques, mainly intensified-charge coupled device
(iCCD) imaging and optical emission spectroscopy. The
temporal development of the plasma column is studied
during the pulse and a special emphasis is put on the
breakdown interval, i.e., the first few hundred
microseconds. The gas temperature is also derived using
the assumption that the He atoms are in thermodynamic
equilibrium with the OH molecules present as impurities
in the gas, the rotational temperature of OH being derived
from its molecular band emission at 306 nm.
2. Experimental set-up
The experimental set-up used is presented in Fig. 1.
The microwave power (2450 MHz) is provided by a
Gerling GL114 microwave generator and is coupled to the
plasma through a surfaguide wave launcher surrounding a
dielectric tube. The system is pulsed at 50 Hz with a duty
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cycle of 5%. A movable plunger ensure optimal coupling
between the transmission line and the plasma. Helium is
99.999% pure and the fused silica tubes are 6 and 10 mm
of outer diameter.
Fig. 1. Experimental set-up of PSWDs in helium gas.
The gas pressure range explored in this paper varies
from 1 to 50 Torr and the average absorbed power from 8
to 30 Watts. For time resolved-imaging, an iCCD camera
1
from Andor Technology (520-18F-01, 1024×256 pixels)
was used and, for optical emission spectroscopy, a Jobin
Yvon spectrometer (Triax 550 monochromator) with
3600 lines mm‾1 grating blazed in UV (i.e., 200 - 450 nm)
was employed. The latter is used to acquire OH
molecular band emitted at 306.4 nm (𝐴2 Σ + ; 𝜈 = 0 −
𝑋 2 Π; 𝜈′ = 0) in order to determine its rotational
temperature.
3. Results and discussion
3.1 iCCD imaging
The plasma column development is studied by time
resolved imaging.
Depending on tube dimension,
absorbed power and gas pressure, different physical
phenomena can be identified. The plasma column is
shown in Fig. 1 for three values of absorbed power in the
case of the smaller tube diameter (6 mm-outer) and during
the first tens of µs. First, for any absorbed power, the
plasma column is symmetrical with respect to the tube
axis along the whole pulse duration (i.e., during 1 ms).
The plasma is ignited in the tube in the wave launcher
region and it fills only a short part of the tube (Fig. 2a @
396 µs, Fig. 2b @ 216 µs and Fig. 2c @ 163 µs). We
note that the plasma is ignited after a given delay time
that decreases with an increase of the absorbed power.
This phenomenon is due to the ‘formative time lag’ which
a physical phenomenon is linking the presence of
electrons caused by the gas breakdown and the formation
of the plasma column [8]. Once the plasma is ignited, the
electromagnetic surface wave can then propagate and
dissipates its energy in the plasma so that the column
length increases.
+
velocity of the ionization front (the ionization front is the
plasma-gas interface perpendicular to the direction of
wave propagation) and the group velocity of the wave, the
surface wave is reflected on the ionization front of the
transient plasma. The interference between the incident
and reflected waves leads to the formation of this bullet
pattern. Its spatial location depends of the absorbed
power and time. For instance, they are ~4 cm, ~9 and
~9.5 cm at 398 µs, 219 and 167 µs and for absorbed
energy of 7 W, 18 and 23 W, respectively. At later time,
the plasma column reaches a steady state and becomes
more homogenous due to diffusion.
At higher tube diameter, the plasma column becomes
non-axisymmetric, especially during the tens of
microseconds after breakdown. The frames presented in
Fig. 3 are obtained in a tube (outer) diameter of 1 cm for a
gas pressure of 5 Torr and an absorbed power of 18 W.
The plasma seems to be attached to the tube wall and
looks similar to the already known phenomenon of
‘surface streamer’ observed in dielectric barrier
discharges [7, 9] even though the underlying physical
mechanism leading to the formation of surface streamers
differs in our case. The observed plasma shape can
mainly be explained by two phenomena. The bullet
plasma results from the surface wave reflection on the
ionization front as mentioned before. The plasma
attachment to the tube wall is due to the interference
between the incident and the reflected wave at the gap of
the wave launcher. The plasma looks always attached to
the side where the incident wave going through the tube
meet with the reflected wave coming back from the
plunger. The increased electric field causes slightly more
ionization on this side and therefore allows quicker
ignition. Once ignited, the wave power is absorbed by the
freshly created electrons and the plasma remains attached
to the tube wall during these tens of microseconds.
2 cm
120 µs
122 µs
124 µs
126 µs
−
Fig. 2. Time evolution of the plasma column. The
pressure is 5 Torr, the absorbed power is a) 7, b) 18 and
c) 23 W. The outer tube diameter is 6 mm.
Several microseconds after ignition, another
phenomenon appears. Indeed, the plasma light becomes
more intense at the column head. It looks similar to the
‘plasma bullet’ phenomenon. In SWDs, the appearance of
this pattern is due to the reflection of the electromagnetic
surface wave. Because of the difference between the
2
Fig. 3. Time evolution of the plasma column. The
pressure is 5 Torr, the absorbed power is 18 W and the
outer tube diameter is 1 cm.
We also studied the time evolution of the plasma
column length as a function of gas pressure and absorbed
power by using the iCCD time resolved photos. The
plasma column length was determined from the plasma
light emission with intensity above an arbitrarily chosen
threshold. At given intervals (10-100 µs) during the
microwave pulse (1 ms), the plasma length was achieved
by averaging 15 values to reduce the statistical errors.
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The plasma column length evolution is presented in Fig. 4
for the following conditions of absorbed power and gas
pressure: {33 W, 5 Torr}, {16 W, 5 Torr} and {33 W, 37
Torr}.
Fig. 4. Time evolution of the plasma column length at
(33 W, 5 Torr), (16 W, 5 Torr) and (33 W, 37 Torr). The
outer tube diameter is 1 cm.
The increase of the plasma column length with time
follows a law of the type {1 − 𝑒𝑒𝑒(−𝑡/𝜏)} and 90% of
the final length being reached after ~400 µs. As
expected, the plasma column length increases with the
absorbed power and decreases with the gas pressure.
The ionization front velocity can be estimated using the
derivative of the column length with respect to time. The
time evolution of this velocity is presented in Fig. 5 for
the following conditions of absorbed power and gas
pressure: {33 W, 5 Torr}, {16 W, 5 Torr} and {33 W, 42
Torr}.
Fig. 5. Time evolution of the ionization front velocity at
(5 Torr, 33 W), (5 Torr, 16 W) and (42 Torr, 33 W). The
outer tube diameter is 1 cm.
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At low gas pressure (5 Torr typically), two regimes of
the ionization front velocity can be identified depending
on the time interval. From breakdown to 50 µs, the
velocity decreases rapidly from ~1500 to ~400 m s‾1.
Afterwards, the velocity decreases slowly to reach ~200
m s‾1 at 400 µs after the breakdown. Changing the
absorbed power from 8 to 33 W does not strongly change
the velocity behaviour except that the breakdown delay
decreases by increasing the power. However, the pressure
has a quite different influence on the ionization front
velocity especially above about 30 Torr. Indeed, at
42 Torr, the plasma is ignited in a short region close to the
wavelauncher and its length increases at a velocity of
~25 m s‾1 in the first microseconds. During the following
50 µs after breakdown, the velocity increases to reach
~400 m s‾1. Then, as at lower pressure, the ionization
front velocity decreases slowly to reach the ~200 m s‾1 at
400 µs after breakdown.
3.2 Temperature
The gas temperature is an important plasma parameter.
Optical methods can be used to obtain it. They consist in
analysing the rotational structure of some molecular
bands present in the discharge. The more employed
molecules are OH, NH, N 2 and N 2 + [10] when at least
one of these molecules is present in the plasma, even as
impurity. The rotational distribution of the intensity is
generally considered in thermal equilibrium with the
neutral gas temperature provided the collisional
exchanges are fast enough. Among the mentioned
molecules, OH is considered as the best temperature
probe [11, 12] due to the way that this molecule is created
and excited to the higher level, to the rotational constant
of the molecule and the number of collisions that the
molecule should do with the gas atoms to reach thermal
equilibrium. Therefore, the spectrum of OH molecules in
the spectral range 307.5-310 nm (𝐴2 Σ + − 𝑋 2 Π) was used
to determine the rotational temperature.
These
measurements were performed at different time delays
and at various axial positions z from the wavelauncher.
Note that the position of the wavelauncher corresponds to
z = 0.
The time evolution of the temperature at different axial
positions is presented in Fig. 6 for a gas pressure of 5
Torr, an absorbed power of 33 W and an outer tube
diameter of 1 cm.
Just after breakdown, the gas (rotational) temperature
(375 - 450 K) is close to the room temperature (300 K). It
increases rapidly to reach its steady-state value at the end
of the microwave power pulse. The higher steady-state
temperature is achieved close the wave launcher and it
decreases by moving away from this position. For
instance, by moving from 2 cm to 22 cm, the steady-state
temperature decreases from 700 to 450 K. We note that
close to the wave launcher, the steady-state temperature is
slower to be attained as compared to the other positions
along the plasma column. The temperature corresponding
to the appearance of the plasma at a given position
decreases with the distance from the wave launcher,
3
Fig. 6. Time evolution of the OH rotational temperature
at different axial positions. The pressure is 5 Torr, the
absorbed power is 33 W and the outer tube diameter is
1cm.
ranging from ~450 K at 2 cm to ~375 K at 22 cm. This
can be explained by the dissipation of the available power
as the wave travels along the plasma column. The plasma
stops once the power becomes insufficient to create the
critical electron number to sustain the discharge.
4. Conclusion
The investigation of pulsed discharges in helium at
pressure range of 1 - 50 Torr sustained by electromagnetic
surface waves were presented in this paper. The
discharge was characterized by time-resolved iCCD
imaging and time resolved optical emission spectroscopy.
It has been observed that the plasma column is structured,
especially during the tens microseconds after breakdown.
For a tube of 6 mm outer diameter, a plasma bullet is
formed. It propagates at a velocity of 1-10 km s‾1
depending on the absorbed power. The bullet formation
is explained by the reflection of the electromagnetic
surface wave on the ionization front. When the tube
diameter increases, the plasma column becomes nonaxisymmetric and the bullet is attached to the tube wall.
We note that for a comparable average absorbed power
and gas pressure, the ionization front wave velocity is one
order of magnitude lower in the larger tube.
The time evolution of the gas temperature was deduced
from the rotational structure of an emission band of OH
molecules (present as impurity). Near breakdown, the
temperature is close to the room one and it increases to
reach a steady state at ~100 µs. By moving away from
the wavelauncher, the steady-state temperature decreases
quasi linearly with the axial position.
The perspective of this work is to determine the density
of metastable (in 23S and 21S levels) atoms in the
discharge using time- and space-resolved absorption
spectroscopy.
4
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